Fiber lasers, compared to most other types of solid state lasers, are able to exhibit very high optical gains (e.g. 20-40 dB), making them useful for amplifying low-power sources such as single mode, single-frequency semiconductor lasers, and the attenuated signals found in long distance (e.g., a few kilometers) fiber-based communications systems. In addition, with the high gains they are able to operate efficiently over large fraction of their emission spectrum, on the order of the span from 50% on either side of the peak gain wavelength.
These properties are due to the ability of the fiber laser to maintain a high intensity of pump power in the core of the fiber for relatively modest (e.g., 10 mW and higher) optical pump power levels. The high intensity leads to saturation of the absorption process and a resultant high population inversion over a long length of fiber, producing a high gain. This is true even for laser transitions that have a small gain cross section (e.g., on the order of 10−21 cm2), such as that of the Er3+ ion transition in the 1550-nm wavelength region, widely used for fiber-based communications.
For wavelengths well away from that of the peak gain cross section (e.g., past the point of 50% of the peak), it is in principle possible in conventional fiber lasers to get high gain by further increasing the pump power and thereby increasing the length of fiber with a high inversion level. However, gain at or close to the peak wavelength increases as well, and eventually becomes so high that, even when extraordinary measures are taken to prevent optical feedback and the resultant unwanted lasing rather than amplification, simply the amplification of background noise in the fiber (Amplified Spontaneous Emission, or ASE) leads to a saturation of the maximum gain available. Adding pump power results in an addition to the ASE power generated by the fiber, and minimal increase in the gain. As a result, gain at the desired wavelength away from the peak can fall well below that desired. The effect of ASE or unwanted oscillation also limits the ability of a fiber laser to generate or amplify high-energy, low-duty cycle pulses at wavelengths far from those of the highest gain cross section, since ASE or oscillation can build up between pulses.
Techniques to counter the effects discussed above have included the use of special fiber core designs that create high loss for wavelengths around the gain peak, and low loss at the desired wavelength, but these designs are complicated and expensive to fabricate.